Fault Adaptive Control of Overactuated Systems Using Prognostic Estimation

Most fault adaptive control research addresses the preservation of system stability or functionality in the presence of a specific failure (fault). This paper examines the fault adaptive control problem for a generic class of incipient failure modes, which do not initially affect system stability, but will eventually cause a catastrophic failure to occur. This risk of catastrophic failure due a component fault mode is some monotonically increasing function of the load on the component. Assuming that a probabilistic prognostic model is available to evaluate the risk of incipient fault modes growing into catastrophic failure conditions, then fundamentally the fault adaptive control problem is to adjust component loads to minimize risk of failure, while not overly degrading nominal performance. A methodology is proposed for posing this problem as a finite horizon constrained optimization, where constraints correspond to maximum risk of failure and maximum deviation from nominal performance. Development of the methodology to handle a general class of overactuated systems is given. Also, the fault adaptive control methodology is demonstrated on an application example of practical significance, an electro-mechanical actuator (EMA) consisting of three DC motors geared to the same output shaft. Similar actuator systems are commonly used in aerospace, transportation, and industrial processes to actuate critical loads, such as aircraft control surfaces. The fault mode simulated in the system is a temperature dependent motor winding insulation degradation.